Nonresonant Raman Control of Material Phases

Important advances have recently been made in the search for materials with complex multi-phase landscapes that host photoinduced metastable collective states with exotic functionalities, such as high-temperature superconductivity, ferroelectricity, or topological properties. In almost all cases so far, the desired phases are accessed by exploiting light-matter interactions via the imaginary part of the dielectric function through above-bandgap or resonant mode excitation. Nonresonant Raman excitation of coherent modes has been observed experimentally and proposed as a means of dynamically controlling material functions. However, the atomic excursion driven by this approach has been perturbative, and these prospects have been hindered by the concomitant excitation of carriers and subsequent heating-induced sample damage. Here, we demonstrate that it is possible to overcome this challenge by employing nonresonant ultrashort pulses with low photon energies significantly below the bandgap. We first achieve this in a prototypical ferroelectric, lithium niobate, using mid-infrared pulse excitation and concurrently monitoring the lattice dynamics using femtosecond stimulated Raman scattering and second harmonic generation. Large-amplitude ferroelectric soft mode displacements driven by nonresonant Raman excitation can reverse the Raman polarizability sign and the second harmonic phase, indicating a ferroelectric reversal. We extend this to tin selenide, a material with complex energy landscapes requiring simultaneous excitation of multiple modes to trigger phase transformation. Using time-domain Raman scattering and time-resolved X-ray diffraction to monitor mid-infrared-excited tin selenide, we observe the suppression of <i>A</i>_g Raman modes beyond a critical MIR field strength, indicating a new phase formation. Reconstructed atomic displacements from structural factor changes show distinct lattice dynamics compared to heat or carrier excitation. Further corroborated with first-principle calculations, this discovery introduces a novel phase control method that goes beyond the conventional resonant excitation approach and unlocks exciting possibilities for facile manipulation of phases and chemical reactivity with complex energy landscapes at reduced energy consumption and ultrafast operation speeds.